Allow configuration of Jinja extensions

[dannyadair]

In render.py's create_env():

    env = jinja2.Environment(
        loader=jinja2.FileSystemLoader(str(templates_dir)),
        autoescape=jinja2.select_autoescape(),
        extensions=["jinja2.ext.do"],
    )

The "do" is needed for one of our messy templates.
Would like to remove that again.
But what if someone else needs it?

Add a YML config option (on any level) to set the extensions.

P.S.: Maybe also "autoescape"? Will wait for feature request (templates have the safe filter).

[dannyadair]

Implemented in c925de0